1. Field of the Invention
The present invention relates to a voltage-controlled oscillator capable of varying its oscillation frequency by voltage control and a communication device employing the same.
2. Description of the Prior Art
In general, a voltage-controlled oscillator employing a variable capacitive element such as a varactor diode is proposed. FIG. 14 is a plan view of a conventional voltage-controlled oscillator disclosed in Japanese Patent Laying-Open No. 57-87209 (1982).
Referring to FIG. 14, a metal-semiconductor field-effect transistor (MESFET; hereinafter referred to as a transistor) 55 is formed on a dielectric substrate 51. Microstrip lines 52, 53 and 54 connected with a source electrode S, a gate electrode G and a drain electrode D of the transistor 55 respectively are also formed on the dielectric substrate 51. A gate-side stabilizing resistor 57 is connected to an end of the microstrip line 53.
A dielectric resonator 56 is arranged on the dielectric substrate 51 to be electromagnetically coupled with the microstrip lines 53 and 54. A further microstrip line 59 is formed on the dielectric substrate 51 to be electromagnetically coupled with the dielectric resonator 56. This micro strip line 59 has a length half the wavelength corresponding to a prescribed frequency (this wavelength is hereinafter referred to as a half wavelength), and approximates to the dielectric resonator 56 on its middle point. Thus, the middle point of the microstrip line 59 defines a node 70 with the dielectric resonator 56.
A first end 59a of the microstrip line 59 is open. A further microstrip line 60 is arranged on a second end 59b of the microstrip line 59 through a varactor diode 58. The microstrip line 60 has a length quarter the wavelength corresponding to the prescribed frequency (this length is hereinafter referred to as a quarter wavelength).
A cathode C of the varactor diode 58 is connected to the second end 59b of the microstrip line 59, and an anode A is connected to a first end 60a of the microstrip line 60. A second end 60b of the microstrip line 60 is open.
In the voltage-controlled oscillator shown in FIG. 14, the transistor 55 amplifies a small microwave signal generated on the gate electrode G and outputs the amplified microwave signal to the drain electrode D. The microstrip lines 54 and 53 and the dielectric resonator 56 form a band-pass filter. The microwave signal output to the drain electrode D is positively fed back to the gate electrode G through this band-pass filter. Thus, microwave power oscillating at a constant oscillation frequency is obtained. This oscillation frequency depends on the resonance frequency of the dielectric resonator 56.
A control voltage is applied across the cathode C and the anode A of the varactor diode 58. The capacitance value of the varactor diode 58 varies with the control voltage applied across the cathode C and the anode A.
The dielectric resonator 56 and the microstrip line 59 are electromagnetically coupled with each other, and the resonance frequency of the dielectric resonator 56 varies with the capacitance value of the varactor diode 58. Therefore, this voltage-controlled oscillator can vary the oscillation frequency by changing the control voltage applied across the cathode C and the anode A.
In the aforementioned conventional voltage-controlled oscillator, the microstrip line 59 has the length corresponding to the half wavelength and the open first end 59a, and hence the central node 70 is shorted (in a shorted state) in a high-frequency manner while the second end 59b is open (in an open state) in a high-frequency manner. The microstrip line 60 has the quarter wavelength and the open second end 60b, and hence the first end 60a is shorted (in a shorted state) in a high-frequency manner.
In such a structure of the voltage-controlled oscillator, the dielectric resonator 56 is electromagnetically coupled with the varactor diode 58 through the microstrip line 59 and hence the microwave power oscillating at the constant oscillation frequency partially reaches the varactor diode 58. The anode A of the varactor diode 58 is grounded in a high-frequency manner so that the potential thereof is regularly kept at zero. On the other hand, the cathode C of the varactor diode 58 is open in a high-frequency manner, and hence a voltage resulting from the microwave power is superposed on the control voltage. Thus, the following potential difference Vva is caused between the cathode C and the anode A of the varactor diode 58:
Vva=Vc+Vpoxc2x7sin(2xcfx80ft)
where Vc represents the control voltage applied across the cathode C and the anode A of the varactor diode 58, f represents the oscillation frequency, Vpo represents the amplitude of the voltage resulting from the microwave power oscillating at the oscillation frequency f and t represents the time.
As understood from the above equation, the potential difference Vva between the cathode C and the anode A of the varactor diode 58 fluctuates, followed by fluctuation of the capacitance value of the varactor diode 58. Consequently, the oscillation frequency f also fluctuates to deteriorate phase noise characteristics of oscillating waves as a result.
The capacitance value of the varactor diode 58 is nonlinear with respect to the voltage. When the potential difference between the cathode C and the anode A of the varactor diode 58 having such nonlinearity fluctuates, baseband noise of the transistor 55 and the varactor diode 58 is converted to a frequency close to the oscillation frequency f, to deteriorate the phase noise characteristics of the oscillating waves as a result. As the dielectric resonator 56 and the microstrip line 59 are strongly coupled with each other, the part of the microwave power reaching the varactor diode 58 increases to more remarkably deteriorate the phase noise characteristics.
In order to reduce such deterioration of the phase noise characteristics caused by the voltage superposed on the controlled voltage for the varactor diode, a countermeasure of connecting two varactor diodes in parallel with each other in opposite polarity is proposed as disclosed in Japanese Patent Laying-Open No. 4-223601 (1192), for example.
However, this structure requires two varactor diodes having completely identical voltage dependency of capacitance values. If the capacitance values of the varactor diodes are asymmetrical with respect to a control voltage, the composite capacitance value of the two varactor diodes fluctuates due to fluctuation of potential differences between cathodes and anodes. Thus, this means cannot solve the problem of fluctuation of the oscillation frequency.
An object of the present invention is to provide a voltage-controlled oscillator reduced in deterioration of phase noise characteristics.
Another object of the present invention is to provide a communication device having a high communication quality resulting from a reduction in deterioration of phase noise characteristics.
A voltage-controlled oscillator according to an aspect of the present invention comprises an oscillation part performing oscillation, a resonance circuit resonating with the oscillation frequency of the oscillation part and a modulation circuit for modulating the oscillation frequency of the oscillation part within an oscillation band by changing the resonance frequency of the resonance circuit, while the modulation circuit includes a coupling part coupled with the resonance circuit in a high-frequency manner and a variable capacitive element having a pair of electrodes subjected to application of a control voltage, and the input impedance of the coupling part as viewed from the side of the resonance circuit at a frequency within the oscillation band is substantially set in a shorted state while one of the pair of electrodes of the variable capacitive element is connected to the coupling part.
In this voltage-controlled oscillator, the resonance circuit resonates with the oscillation frequency of the oscillation part. When changing the control voltage applied across the pair of electrodes of the variable capacitive element in the modulation circuit, the resonance frequency of the resonance circuit changes to modulate the oscillation frequency of the oscillation part within the oscillation band.
In this case, the input impedance of the coupling part as viewed from the side of the resonance circuit at the frequency within the oscillation band is substantially set in a shorted state. Also when power oscillating at the oscillation frequency partially reaches the coupling part of the modulation circuit, therefore, fluctuation of the potential of the coupling part is suppressed. Thus, the potential difference between the pair of electrodes of the variable capacitive element is kept constant and the capacitive element is prevented from fluctuation of its capacitance value. Therefore, the oscillation frequency does not fluctuate, not to deteriorate phase noise characteristics as a result.
The potential difference between the pair of electrodes of the variable capacitive element is kept constant, whereby baseband noise of the oscillation part and the variable capacitive element is prevented from being converted to a frequency around the oscillation frequency due to nonlinearity between the capacitance value of the variable capacitive element and the voltage, not to deteriorate the phase noise characteristics of oscillating waves as a result. Accordingly, a deterioration in a communication quality resulting from a deterioration in phase noise characteristics in a communication device.
The resonance circuit may include a resonance element formed by a cylindrical dielectric material or a discoidal conductive material, and the coupling part may be electromagnetically coupled with the resonance circuit.
The modulation circuit may further include a transmission line having an open end and the coupling part, and the length between the open end and the coupling part of the transmission line may be approximately set to odd times a quarter of an effective wavelength corresponding to a frequency within the oscillation band. Thus, the input impedance of the coupling part as viewed from the side of the resonance circuit at the frequency within the oscillation band is substantially shorted.
When the length of the transmission line increases, the occupied area as well as transmission loss also increase to lower the Q (quality factor) of the circuit and deteriorate noise characteristics. Therefore, the length between the open end and the coupling part of the transmission line is preferably set around quarter the effective wavelength corresponding to the frequency within the oscillation band.
The length between the open end and the coupling part of the transmission line may be set shorter than the odd times a quarter of the effective wavelength corresponding to the frequency within the oscillation band. Thus, when the transmission line is effectively extended due to capacitance present on the open end of the transmission line, the input impedance of the coupling part as viewed from the side of the resonance circuit at the frequency within the oscillation band is substantially shorted.
The length between an open end of the transmission line effectively extended due to capacitance present on the open end of the transmission line and the coupling part may be set to odd times a quarter of the effective wavelength corresponding to the frequency within the oscillation band. Thus, when capacitance is present on the open end of the transmission line, the input impedance of the coupling part as viewed from the side of the resonance circuit at the frequency within the oscillation band is substantially shorted.
The variable capacitive element may be a varactor diode. Further, the transmission line may be a microstrip line.
A voltage-controlled oscillator according to another aspect of the present invention comprises an oscillation part performing oscillation, a resonance circuit resonating with the oscillation frequency of the oscillation part and a modulation circuit for modulating the oscillation frequency of the oscillation part within an oscillation band by changing the resonance frequency of the resonance circuit, while the modulation circuit includes a pair of coupling parts coupled with the resonance circuit in a high-frequency manner and a variable capacitive element having a pair of electrodes subjected to application of a control voltage, and the pair of electrodes of the variable capacitive element are connected to the pair of coupling parts respectively.
In this voltage-controlled oscillator, the resonance circuit resonates with the oscillation frequency of the oscillation part. When changing the control voltage applied across the pair of electrodes of the variable capacitive element in the modulation circuit, the resonance frequency of the resonance circuit changes to modulate the oscillation frequency of the oscillation part within the oscillation band.
The pair of coupling parts of the modulation circuit are coupled with the resonance circuit in a high-frequency manner. When power oscillating at a constant oscillation frequency partially reaches the pair of coupling parts of the modulation circuit, therefore, a voltage resulting from the oscillation power is equally supplied to the pair of electrodes of the variable capacitive element. Thus, the potential difference between the pair of electrodes of the variable capacitive element is kept constant and the capacitance of the variable capacitive element does not fluctuate. Therefore, the oscillation frequency does not fluctuate, not to deteriorate phase noise characteristics as a result.
The potential difference between the pair of electrodes of the variable capacitive element is kept constant, whereby baseband noise of the oscillation part and the variable capacitive element is prevented from being converted to a frequency around the oscillation frequency due to nonlinearity between the capacitance value of the variable capacitive element and the voltage, not to deteriorate phase noise characteristics of oscillating waves as a result.
The input impedances of the pair of coupling parts as viewed from the side of the resonance circuit at a frequency within the oscillation band may be substantially set in shorted states respectively.
In this case, the potentials of the pair of coupling parts are inhibited from fluctuation and the potential difference between the pair of electrodes of the variable capacitive element is kept constant also when different voltages are supplied to the pair of coupling parts of the modulation circuit due to the oscillation power. Consequently, the variable capacitive element is reliably prevented from fluctuation of the capacitance value.
The resonance circuit may include a resonance element formed by a cylindrical dielectric material or a discoidal conductive material, and the pair of coupling parts may be electromagnetically coupled with the resonance element.
The modulation circuit may further include a pair of transmission lines, the first one of the pair of transmission lines may have an open end and the first one of the pair of coupling parts, and the second one of the pair of transmission lines may have an open end and the second one of the pair of coupling parts.
The length between the open end and the first coupling part of the first transmission line may be equal to the length between the open end and the second coupling part of the second transmission line. In this case, the potential difference between the pair of electrodes of the variable capacitive element can be kept constant by equally setting coupling between the resonance circuit and the first coupling part and coupling between the resonance circuit and the second coupling part.
The length between the open end and the first coupling part of the first transmission line may be different from the length between the open end and the second coupling part of the second transmission line. In this case, the potential difference between the pair of electrodes of the variable capacitive element can be kept constant by differently setting coupling between the resonance circuit and the first coupling part and coupling between the resonance circuit and the second coupling part.
The resonance circuit may include a distributed constant element. In this case, a distributed constant line forms the resonance circuit.
The resonance circuit may include a lumped constant element. In this case, a lumped constant circuit forms the resonance circuit.
The variable capacitive element may be a varactor diode. Further, the transmission lines may be microstrip lines.
A communication device according to still another aspect of the present invention comprises a local oscillator including a voltage-controlled oscillator generating a reference signal and a frequency converter mixing the reference signal generated by the local oscillator with a transmitted signal or a received signal thereby converting the frequency of the transmitted signal or the received signal to a prescribed frequency, while the voltage-controlled oscillator includes an oscillation part performing oscillation, a resonance circuit resonating with the oscillation frequency of the oscillation part and a modulation circuit for modulating the oscillation frequency of the oscillation part within an oscillation band by changing the resonance frequency of the resonance circuit, the modulation circuit includes a coupling part coupled with the resonance circuit in a high-frequency manner and a variable capacitive element having a pair of electrodes subjected to application of a control voltage, and the input impedance of the coupling part as viewed from the side of the resonance circuit at a frequency within the oscillation band is substantially set in a shorted state while one of the pair of electrodes of the variable capacitive element is connected to the coupling part. Accordingly, a deterioration in a communication quality resulting from a deterioration in phase noise characteristics in the communication device.
In the voltage-controlled oscillator of this communication device, the input impedance of the coupling part as viewed from the side of the resonance circuit at the frequency within the oscillation band is substantially set in a shorted state, whereby the coupling part of the modulation part is inhibited from potential fluctuation also when power oscillating at the oscillation frequency partially reaches the coupling part. Thus, the potential difference between the pair of electrodes of the variable capacitive element is kept constant and the variable capacitive element is prevented from fluctuation of the capacitance value. Therefore, the oscillation frequency does not fluctuate, not to deteriorate phase noise characteristics as a result.
The potential difference between the pair of electrodes of the variable capacitive element is kept constant, whereby baseband noise of the oscillation part and the variable capacitive element is prevented from being converted to a frequency around the oscillation frequency due to nonlinearity between the capacitance value of the variable capacitive element and the voltage, not to deteriorate phase noise characteristics of oscillating waves as a result.
A communication device according to a further aspect of the present invention comprises a local oscillator including a voltage-controlled oscillator generating a reference signal and a frequency converter mixing the reference signal generated by the local oscillator with a transmitted signal or a received signal thereby converting the frequency of the transmitted signal or the received signal to a prescribed frequency, while the voltage-controlled oscillator includes an oscillation part performing oscillation, a resonance circuit resonating with the oscillation frequency of the oscillation part and a modulation circuit for modulating the oscillation frequency of the oscillation part within an oscillation band by changing the resonance frequency of the resonance circuit, the modulation circuit includes a pair of coupling parts coupled with the resonance circuit in a high-frequency manner and a variable capacitive element having a pair of electrodes subjected to application of a control voltage, and the pair of electrodes of the variable capacitive element are connected to the pair of coupling parts respectively.
In the voltage-controlled oscillator of this communication device, the pair of coupling parts of the modulation circuit are coupled with the resonance circuit in a high-frequency manner. When power oscillating at a constant oscillation frequency partially reaches the pair of coupling parts of the modulation circuit, therefore, a voltage resulting from the oscillation power is equally supplied to the pair of electrodes of the variable capacitive element. Thus, the potential difference between the pair of electrodes of the variable capacitive element is kept constant and the capacitance value of the capacitive element does not fluctuate. Therefore, the oscillation frequency does not fluctuate, not to deteriorate phase noise characteristics as a result.
The potential difference between the pair of electrodes of the variable capacitive element is kept constant, whereby baseband noise of the oscillation part and the variable capacitive element is prevented from being converted to a frequency around the oscillation frequency due to nonlinearity between the capacitance value of the variable capacitive element and a voltage, not to deteriorate phase noise characteristics of oscillating waves as a result. Accordingly, a deterioration in a communication quality resulting from a deterioration in phase noise characteristics in the communication device.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.